A typical gas turbine engine includes a turbine module comprising a hub, which is rotatable about an engine axis, and an array of blades projecting radially outwardly from the hub. The turbine module also includes a case with a set of circumferentially distributed outer airseals mounted thereon so that the case and the airseals circumscribe the blade array. The airseals define the radially outermost boundary of a flowpath for a working medium fluid. The airseals are often made of a nickel base alloy or superalloy. The airseals are spaced radially from the radially outermost tips of the blades to define a radial clearance gap. During engine operation, mechanical and thermal influences can cause the radial size of the gap to change. It is desirable to control the size of the gap by, for example, impinging cool air on the case to decrease its diameter or withholding the cool air to allow the case diameter to expand. Although such control can be open loop, it may also be desirable to effect closed loop control of the clearance gap. Closed loop control requires a clearance sensing system for measuring the radial size of the gap.
One suitable clearance sensing system is a microwave system that uses microwave radiation to determine the radial size of the clearance gap. Such a system includes a waveguide residing radially outboard of the outer airseals and an associated electronics package and algorithms for signal processing. One or more selected outer airseals includes a subassembly comprising a microwave transparent ceramic window installed in a window frame made of a nickel base alloy or superalloy. The subassembly occupies an opening in the selected outer airseal and is hermetically joined to the outer airseal to prevent leakage through the opening. When the turbine is assembled, the window is axially and circumferentially aligned with the waveguide so that the clearance sensing system can detect the proximity of the blade tips. Alternatively, the ceramic window may be joined directly to the airseal without an intervening window frame. Either configuration presents the problem of joining the ceramic window to a nonceramic host, either the nickel base window frame or the nickel base outer airseal. The joint must be able to withstand the challenging conditions, including extreme temperatures, encountered in the turbine module of a gas turbine engine.
What is needed is convenient, cost effective method of forming a secure joint between a ceramic component and a nonceramic host.